Conventional devices for forming an image on a recording medium such as paper or an OHP sheet have been proposed in a form having a mounted printhead that operates based on various printing techniques. Examples of these printing techniques are the wire dot, thermosensitive, thermal transfer and ink-jet techniques. All of these techniques generally employ a printhead in which a plurality of print elements are integrated in order to raise printing efficiency.
In an arrangement of this type, it is difficult to avoid band-like density unevenness caused by variations in the spacing between the print elements and by the mechanical precision with which the printhead and print medium are moved. More specifically, with the ink-jet technique, minute stripes are produced by slight differences in ink discharge direction and quantity from one ink discharge nozzle, i.e., print element, to the next, and band-like density unevenness is produced at intervals equivalent to the amount of movement of the print medium owing to errors in nozzle spacing and in the amount of movement of the print medium.
A head shading correction such as disclosed in the specification of Japanese Patent Application Laid-Open No. 5-069545 has been used as a method of correcting such density unevenness. In brief, this method involves outputting a test chart by an image forming apparatus, reading the characteristic of unevenness contained in the results output by an image reading unit such as a scanner, correcting the image data so as to cancel out this unevenness, and supplying the corrected data to the image forming apparatus.
However, when use is made of a method of correcting density unevenness using the head shading correction, the number of tone levels of the input image is reduced by the correction and output time is prolonged.